New Coating Traps Light In Solar Cells Like Sound In A Whispering Gallery

Artist's representation of a solar panel coated with optical whispering galleries to increase light absorption. The actual beads are far too small to be seen without powerful microscopes. K. Dill, D. Ha, G. Holland/NIST

The quest for better ways to harness sunlight continues with two teams announcing new methods to increase solar cell efficiency and flexibility.

Even the best solar cells don't capture every photon that falls on them, leading to the search for coatings that will improve the situation. This has usually been done by back-coating the cells, in order to reflect photons that pass through to give the cell a second chance. However, Dr Dongheon Ha of the US National Institute of Standards and Technology has gone the other way, making a coating of tiny beads for the cells' front to produce an optical “whispering gallery”.

Whispering galleries occur when curved walls shape sound waves and focus them on a particular location. In domes such as St Paul's Cathedral, an effect is created where, when you stand in the right spot, faint sounds from elsewhere in the dome sound as if they were made very nearby.

Glass beads a few billionths of a meter across have been used to do the same thing to light for about 10 years, but have only recently been applied to solar work. Ha created a coating that bends light before releasing it to fall on a gallium arsenide solar cell. The wavelength of light captured depends on the bead's size, so combining multiple sizes in a coating allows many parts of the spectrum to be harnessed.

When tested under laser light, the coated cells produced 20 percent more electricity than without the coating, Ha announced in Nanotechnology.

Although it's possible to increase solar cells' photon absorption by sculpting them at a similar tiny scale, the increased surface area can undermine the benefits. Additional opportunity for electrons to be captured by so-called “holes” reduces the voltage produced at the cell's contacts and therefore the energy generated. Making the cells themselves smooth and coating them with the optical whispering galleries minimizes voltage loss. The beads can be manufactured in solution, with droplets smeared across cell faces, which Ha called “an inexpensive process... compatible with mass production,” in a statement

Meanwhile, a different program has found ways to make highly flexible organic photovoltaics (OPVs) more efficient. These sorts of solar cells are never likely to offer the best value for large solar farms, but could fill a large niche where cells need to be able to bend, for example when attached to tents, backpacks, or even clothes.

Professor Takao Someya of the University of Tokyo has announced in the Proceedings of the National Academy of Sciences the creation of ultra-flexible OPVs with 10 percent efficiencies. These can be hot-melted onto fabrics, survive temperatures above 100ºC (312ºF), and lost less than a fifth of their efficiency after accelerated testing.

A highly efficient solar cell, capable of surviving high temperatures, stuck to fabric and able to bend with it.  Xiaomin Xu and Kenjiro Fukuda


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